LECTURE XIII.] HISTORY OF CHEMISTRY. 253 



two different specific volumes may be assigned to oxygen, 

 dependent upon whether it exists within or outside the radical. 

 Hence it would not by any means be a matter of indifference, 

 in calculating the specific volumes of aldehydes and of ketones, 

 whether they were referred to the hydrogen or to the water 

 type ; whereas Gerhardt had declared both to be admissible. 9 

 Kopp's rule agreed with the first alternative only. Kopp 

 draws attention to this, and points out that it is exactly in this 

 way that propyl aldehyde is distinguished from the isomeric 

 allyl alcohol : 



C.H.01 C 3 H 6 \ 



H J H J U 



Propyl aldehyde. Allyl alcohol. 



It may certainly be looked upon as a very important sign 

 of the times that the chemists of Gerhardt's school are now 

 forced, upon physical grounds, to attach greater value to their 

 formulae and speculations than had hitherto appeared to them 

 to be justified. It is true that no special stimulus was now 

 required. Indeed it would appear that Couper had already tried 

 to write real constitutional formulae. Assuming the quadri- 

 valence of carbon (quite independently of Kekule) Couper 

 had pointed out how the existence of a great number of 

 organic compounds might be explained. I should like to 

 compare this paper of Couper's 10 with that of Kekule, 

 published a short time previously, in order to show how these 

 two investigators, starting from different points of view, arrive 

 at very similar results. Kekule, in recognising and explaining 

 the real essence of the types, hit upon the quadrivalence of 

 carbon and the mutual union of the atoms. Couper, on the 

 other hand, rejects the types because they do not appear to 

 him to satisfy the philosophical requirements which are 

 essential to a theory. According to him, Gerhardt's system 

 rests upon general statements from which individual cases are 

 deduced ; whereas he declares the opposite method to be the 



9 Gerhardt, Traite. 4, 632 and 80$. 10 Comptes Rendus. 46, 1157 

 Ann. Chim. [3] 53, 469. 



